THE GROWTH OF BACTERIA IN MILK 83 



but belong more commonly to the type which liquefies gelatin, 

 digests casien and produces general putrefaction and decomposi- 

 tion. In the end, unless it dries, they cause the putrefac- 

 tion and decay of the product, a change that would have oc- 

 curred in the milk earlier if it had not been for the protecting 

 action of the lactic bacteria. These further changes, however, 

 need not be considered at this point. 



The phases above described occur normally in milk that is 

 preserved at a temperature not above 70. If the temperature 

 is higher, the changes will be quite different. The different 

 stages will not only follow each other more rapidly, but the 

 souring of the milk will be produced by the growth of the dif- 

 ferent kinds of bacteria, and the resulting material will be of 

 a totally different nature. It may be full of gas bubbles, fty 

 may have vile odors, but, in most cases, it will be a product 

 which dairymen will recognize at once as abnormal and un- 

 desirable. 



While the phases above outlined may be taken as illustrating 

 the general changes in milk, they can only be looked upon as 

 representing an average. Different samples will differ very 

 much. Sometimes the milk will contain bacteria more vigorous 

 than the lactic acid type, and these will gain the upper hand, 

 as, for example, when the slimy bacteria are present. More- 

 over, the various bacteria in a sample of milk have influence 

 upon each other. Sometimes one species will hasten the growth 

 of another species, as shown by Marshall, 1 and sometimes one 

 will delay the growth of others. The phenomena are so widely 

 varied with different samples of milk that any description will 

 be only approximate. The course as given above represents 

 the usual series of events when milk contains the normal bac- 

 teria and is kept at about 60 to 70. 



1 Marshall. Cent. f. Bact., II., p. 400, 1905. 

 Bouska. Rev. Gen. d'Lait, III., p. 1, 1903. 



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